X-ray examinations normally have been conducted by putting the examined organ between an X-ray sensitive film and an X-ray source and radiating X-rays. Film exposed in this way being developed in a normal manner, will show different blackening dependent upon the type and amount of tissue between the X-ray source and the film.
The method among other things has a drawback in that it is not simply reproducible. Images made during what may seem to be the same conditions may present very different blackening and resolution of the interesting areas of the organ being examined. Further the method is labor and time consuming as the result is achieved only after developing of the film.
Within the field of dental X-raying there is now also an application using for example a CCD cell similar to that in an ordinary video camera. This CCD cell is put instead of the X-ray sensitive film in an appropriate manner in the oral cavity of a patient behind the tooth or teeth to be examined, whereby an image direct from the CCD cell may be achieved on a display by means of known technique in the art by using an electronic interface and an ordinary PC computer. This has the advantage that the result is immediately obtained and if this image is not adequate a new exposure may be immediately performed.
In this technique the control electronics of the CCD cell itself has an electrical connection to the X-ray apparatus, such that the electronic interface receives information when the X-ray radiation is activated. This implies that such an arrangement has to particularly be adopted to each available type of X-ray equipment to implement this type of synchronization.
When X-ray images are produced by an image sensing member in the shape of a CCD cell at existing temperatures, e.g. in the oral cavity, dark currents exists which all the time are adding up charges within the CCD cell. These charges are added up so fast that the image sensing member will normally be saturated after one or a few seconds. Therefore, the CCD cell must be emptied of charges before the exposure by X-rays is started. When the X-ray image is fully exposed it must immediately be read out in its totality in order not to loose image information due to the adding of dark currents.
Such a synchronization may be achieved by either activating the X-ray tube short before the scanning is started of for example the CCD cell or initiating scanning from the X-ray arrangement, eventually with a delay such that the radiation source comes to full power. In both cases there is a general demand of considerable interfacing equipment to achieve this. Otherwise the exposure time of the X-ray source must be chosen unnecessarily long to ensure a good result.
According to the state of the art it is not unusual to willingly overexpose a bit to be certain of an acceptable result of the X-ray examination. One problem which then additionally has to be solved is that the accumulated radiation dose of the patient has to be optimized in order to minimize the radiation dose to the patient, which is utterly important from the general standpoint of radiation protection.
A radiation sensor for dental X-ray examination is known from the German utility model G 89 09 398.4.(EP-A1-0 415 075). The imaging device then has a single radiation detector, placed in front of, at the side of or behind the imaging device. The single radiation detector as such may comprise one or more phototransistors having a scintillation layer. One drawback in such an arrangement is that the single radiation detector may be shaded by, for example bone or a tooth when the imaging device is placed into the oral cavity of a patient and thus will detect very little radiation as bone or a tooth will still be very opaque to the kind of X-ray radiation used for dental examination. Additionally the device uses only remote electronics for processing the obtained signals which gives a low efficiency for utilizing the small current generated in the scintillation layer of the radiation sensor.